AZTEC ON ASTE: 1.1-MM CONTINUUM OBSERVATIONS TOWARD THE SMALL MAGELLANIC CLOUD Tatsuya Takekoshi Department of Cosmosciences Graduate School of Science Hokkaido University Collaborators: Tetsuhiro Minamidani, Kazuo Sorai (Hokkaido Univ.) Norikazu Mizuno, Akiko Kawamura (ALMA), Tomoka Tosaki(Joetsu Univ. of education), Yasuo Fukui (Nagoya Univ.) Toshikazu Onishi(Osaka pref. Univ.) Kotaro Kohno(Univ. of Tokyo), Shinya Komugi(ALMA), Monica Rubio(Univ. of Chile) & AzTEC/ASTE team
ISM in low-metallicity Dark gas (e.g. Grenier et al. 2005) H 2 gas component which does not trace by CO observation CO does not trace whole molecular cloud! ~30% is dark in nearby molecular clouds Strongly depend on metallicity (e.g. Wolfire et al. 2010) CO is not good tracer of GMCs in low-metallicity environment? Dark gas HI CO PDR model Hollenbach&Tielens 1999
Key target: the Small Magellanic Cloud Best template for physics of interstellar medium (ISM) and star formation in early universe! Very close! ~60kpc (Hilditch+ 2005) One of the nearest galaxies Resolve GMCs by single dish Low-metallicity ~0.2Z (Larsen+ 2000) Gas-to-dust ratio ~700 (Leroy+ 2007) Active star formation and strong UV field Spitzer Space Telescope (NASA) Blue: 3.6 μm Green: 8.0 μm Red: 24, 70, 160 μm
CO observations toward the SMC SEST Only prominent star forming regions (Rubio et al. 1993 他 ) NANTEN 12 CO(J=1-0) survey toward the SMC (Mizuno et al. 2001) MOPRA High resolution observations toward NANTEN CO clouds (Muller et al. 2010, 2013) Image: SERC-J Contour: 12 CO(J=1-0) (Mizuno et al, 2001)
Dust continuum observations Benefit of dust continuum observation Optically thin Insusceptible to UV field High-speed mapping by Multi-pixel bolometer camera High resolution Efficient exploration of dense gas clumps 1.2mm continuum Bot et al. 2007 mjy/beam Previous SMC observation Only actively star-forming region 1.2mm by SEST/SIMBA (Bot et al. 2007) 870μm by LABOCA/APEX (Bot et al. 2010) Spitzer 160μm (peak of thermal dust emission) The SMC Survey at Rayleigh-Jeans bands are observed by AzTEC/ASTE!
Observations ASTE telescope 10-m diameter Submm telescope Atacama desert, Chile (alt.4800m) beamsize:28 (~8pc)@1.1mm AzTEC Continuum camera Developed by UMASS Wavelength: 1.1mm (270GHz) Bandwidth: 49GHz Pixel number: 144
Survey field 1.1 deg. 1.1 deg., 4 fields Total 4.5 deg 2 Total obs. time: 42 hours 1σ noise lv.: 5-13mJy/beam All star-forming GMCs are detectable! Image: 160μm Contour: CO
Observational result image: AzTEC/ASTE 1.1mm
Comparison with Spitzer 160μm Good spatial correlation with 160μm emission Extended 1.1 mm sources trace cold dust component which dominates dust mass image: Spitzer160μm contour: 1.1mm
Physical Properties Spectral Energy Distribution (SED) analysis Free-free emission (Reynolds 1992) with 3 and 6 cm (Dickel+ 2010) Cold dust emission with1.1mm and 160μm Free-free: 4.8GHz, 8.64GHz continuum(filipovic et al. 1997) 1 component dust temperature and mass assuming β=1.5 (Leroy et al. 2007) Estimate gas mass assuming gas-to-dust ratio ~700 (Leroy et al.2007) 24μm 70μm 160μm 1.1mm 6cm SW 1 T dust = M dust = M gas = 20.8 2.1 K 3 1.2 0.3 10 5 8.3 2.2 10 M M 3cm
Statistical properties of dust clouds Median dust temperature: 20.1 K (~ global big grain, Leroy+ 2007) Median gas mass: 4.4 10 4 M Median radius: 8.7 pc Median H 2 density: 230 cm -3 Median H 2 column density: 8.1 10 21 cm -2 Dust cloud have typical properties of GMCs! Gas mass Radius
CO-dark GMCs About half (21/44) GMCs have no CO detection CO detection rate: total 52% 20% (2/10) in star-less GMCs 62% (21/34) in star-forming GMCs Total 44 SF No SF CO 21 2 CO-less 13 8 CO-detection rate of star-forming GMCs is higher than starless GMCs Prediction from numerical simulation CO abundance is very small before the onset of gravitational collapse of dense regions of a molecular cloud in low-metallicity environment (Glover&Clark 2012) CO-dark starless GMCs might be youngest phase of GMCs
Summary Observation 1.1mm continuum by AzTEC on ASTE SMC survey: total 4.5 deg 2 Result 44 extended object detection Good correlation with 160μm cold dust emission Good correlation with 24μm and CO SED fitting SED fitting Dust temperature ~20K Gas mass 10 4-10 6 M typical GMC mass Star formation and CO CO-detection rate of star-forming GMCs is higher than starless GMCs CO-dark starless GMCs might be youngest phase of GMCs?